1. Field of the Invention
The present invention relates to a low cost flip-flop circuit, and more particularly to a ring oscillator and a phase-locked loop circuit implemented by the low cost flip-flop circuit.
2. Description of the Prior Art
In a conventional N-stage ring oscillator, the delay unit in each stage provides a predetermined delay amount. The delay amount may determine an oscillating frequency of the N-stage ring oscillator. More specifically, the delay amount of the delay unit in each stage depends on an output resistor of the delay unit; however, the resistance of the output resistor may deviate from the desired value due to process variations of the semiconductor manufacturing process. The oscillating frequency of the N-stage ring oscillator may then drift from a desired frequency. In addition, when the N-stage ring oscillator is applied in a phase-locked loop, the characteristic curve of the gain (Kvco) of the N-stage voltage-controlled ring oscillator may also deviate from the desired characteristic curve due to the resistance deviation. To correct for this, the oscillating frequency of the N-stage oscillator should be adjusted to the original-designed frequency after the N-stage oscillator is fabricated, by making the slope of the gain of the voltage-controlled oscillator steeper than the original slope. This allows the oscillating frequency to be adjusted by the input voltage of the voltage-controlled oscillator so it can be calibrated back to the desired frequency band. In order to do this, however, the gain of the voltage-controlled oscillator must be very large, which means the N-stage oscillator becomes very sensitive for the input signal. Moreover, when the gain of the voltage-controlled oscillator is very large, the size (capacitance) of a capacitor of a low-pass filter in the phase-locked loop must also be very large in order to filter out the high frequency signals of the phase-locked loop. Accordingly, the cost of the phase-locked loop is increased. Therefore, how to improve the effects caused by the deviation of the resistor in a delay unit of an N-stage ring oscillator is an urgent problem in this field.